Two analogous methods for estimating the compressive strength of fibrous composites

In this paper, an analogy is made between the solution of micro-buckling of fibrous composites using a three-dimensional model and that of biaxial bending of reinforced concrete short columns. Two approaches are used; the first one uses a reciprocal formula and the second one uses a bilinear approximation to the non-dimensional stresses interaction equation, to estimate the compressive stress in a fibrous composite. The initial misalignment angles of composite fibers, which are the main parameters in the determination of the compressive strength of fibrous composites, are analogous to load eccentricities in concrete columns. The initial misalignment angles in both directions perpendicular to the axis of the fibers are defined by sinusoidal curves. The compressive strength of different fibrous composites, which also depends on the nonlinear shear stress–strain relationship of the matrix material, is estimated using the present approaches. The results obtained in this study agree well with experimental and analytical results available in the literature.     

Ductile deformation mechanisms and designing instructions for integrated woven textile sandwich composites

To suggest designing instructions for integrated woven textile sandwich composites (IWTSCs), anti-crush properties of IWTSC and the corresponding ductile deformation mechanism were investigated. Quasi-static out-of-plane crushing and dynamic impact tests were carried out. Typical deformation curves with a relative stable deformation plateau were obtained from tests. Failure of IWTSC is ductile through coupled compression–shear deformation. An analytical plastic model was proposed to explain ductile mechanism of IWTSC qualitatively, including densification caused by interactions among inclined piles. Combining with qualitative analysis, comparisons between two kinds of IWTSC panels with piles of different density and thickness reveal the key to design a ductile IWTSC.     

Stress-dependence and time-dependence of the post-fatigue tensile behavior of carbon fiber reinforced SiC matrix composites

The major objective of this paper is to phenomenally report the stress-dependence and time-dependence of fatigue damage to C/SiC composites, and to tentatively discuss the effects of the fatigue stress levels and the fatigue cycles on the post-fatigue tensile behavior. Results show that compared with the virgin strength of the as-received C/SiC specimens, the tensile strengths of the as-fatigued specimens after 86,400 cycles were increased by 8.47% at the stresses of 90 ± 30 MPa, 23.47% at 120 ± 40 MPa, and 9.8% at 160 ± 53 MPa. As cycles continued, however, the post-fatigue strength of the composites gradually decreased after the peak of 23.47%, at which the optimal strength enhancement was obtained because the mean fatigue stress of 120 MPa was the closest to thermal residual stress (TRS), and caused TRS relieve largely during the fatigue. Most interestingly, there was a general inflexion appeared on the post-fatigue tensile stress-strain curves, which was just equal to the historic maximum fatigue stress acted upon the as-fatigued specimens. Below this inflexion stress the tensile curves revealed the apparent linear behavior with little AE response, and above that nonlinearity with new damage immediately emitted highly increase rate of AE activities. This ‘stress memory’ characteristic was strongly relevant to damaged microstructures of the as-fatigued composites in the form of the coating/matrix cracks, interface debonding/wear, and fiber breaking, which resulted undoubtedly in reduction of modulus but in proper increase of strength via TRS relief.     

Characterization and biodegradability of polypropylene composites using agricultural residues and waste fish

The objective of this research was to study the potential of waste agricultural residues such as rice-husk fiber (RHF), bagasse fiber (BF), and waste fish (WF) as reinforcing and biodegradable agents for thermoplastic composites. Addition of maleic anhydride grafted polypropylene (MAPP) as coupling agent was performed to promote polymer/fiber interfacial adhesion. Several composites with various polypropylene (PP) as polymer matrix, RHF, BF, WF, and MAPP contents were fabricated by melt compounding in a twin-screw extruder and then by injection molding. The resulting composites were evaluated through mechanical properties in terms of tensile, flexural, elongation at break and Izod notched impact following ASTM procedures. Biodegradability of the composites was measured using soil burial test in order to study the rates of biodegradation of the composites. In general, the addition of RHF and BF promoted an increase in the mechanical properties, except impact strength, compared with the neat PP. According to the results, WF did not have reinforcing effect on the mechanical properties, while it could considerably improve the biodegradation of the composites. It was found that the composites with high content of WF had higher degradation rate. Except impact strength, all mechanical properties were found to enhance with increase in cellulosic fiber loading In addition, mechanical properties and biodegradability of the composites made up using RHF was superior to those of the composites fabricated with BF, due to its morphological (aspect ratio) characteristics.     

Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part I: Polypropylene matrix composites

Platelet-reinforced polymer matrix composites were fabricated by a combined gel-casting and hot-pressing method. Submicrometer-thin alumina platelets were dispersed in a highly diluted grafted maleic anhydride polypropylene solution. Upon cooling, the polymer formed a gel which trapped the platelets in their well separated positions. During subsequent solvent evaporation, the polymer–platelet gel densified and the platelets were oriented horizontally. The dried composites were hot-pressed to further improve the platelet orientation and increase the density of entanglements in the polymer. This method combines several advantages of large scale and lab-scale fabrication methods in that it is fast, simple but also versatile. Composites with platelet volume fractions up to 0.5 were easily fabricated. The maximal achieved yield strength and elastic modulus of the composites were 82% and 13 times higher, respectively, than the values of the polymer alone. The enhancement in the composites mechanical properties was caused by classical load transfer into the platelets as the crystallinity of the polymeric matrix was not affected by the platelets. Alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode enabling large plastic deformation of the composites prior to fracture. At high concentrations of platelets, the strength and stiffness decreased again and the ductility was almost lost due to out-of-plane misalignment of platelets and the increasing number and size of voids incorporated during the fabrication. The designing principles and fabrication method described in this work can potentially be extended to other types of polymers and platelets to create new composites with tailored properties.     

Development of viscoelastic/rate-sensitive-plastic constitutive law for fiber-reinforced composites and its applications. Part I: Theory and material characterization

The viscoelastic/rate-sensitive plastic constitutive law to describe the nonlinear, anisotropic/asymmetric and time/rate-dependent mechanical behavior of fiber-reinforced (sheet) composites was developed under the plane stress condition. In addition to the theoretical aspect of the developed constitutive law, experiments to obtain the material parameters were also carried out for the woven fabric composite based on uni-axial tension and compression tests as well as stress relaxation tests, while the numerical formulation and verifications with experiments are discussed in Part II.     

The static and high strain rate behaviour of a commingled E-glass/polypropylene woven fabric composite

In this paper the effect of strain rate on the tensile, shear and compression behaviour of a commingled E-glass/polypropylene woven fabric composite over a strain rate range of 10⁻³–10² s⁻¹ is reported. The quasi-static tests were conducted on an electro-mechanical universal test machine and a modified instrumented falling weight drop tower was used for high strain rate characterisation. The tensile and compression modulus and strength increased with increasing strain rate. However, the shear modulus and strength were seen to decrease with increasing strain rate. Strain rate constants for use in finite element analyses are derived from the data. The observed failure mechanisms deduced from a microscopic study of the fractured specimens are presented.     

Influence of various starch/hemp mixtures on mechanical and acoustical behavior of starch-hemp composite materials

The starch-hemp composite materials are manufactured from the natural raw materials (water, starch and hemp shives) and a new durable material for construction and building. In this work, experimental investigation was carried out to study the mechanical and acoustical performance of starch-hemp composite materials. The starch-hemp composite materials specimens with five Hemp/Starch ratios (H/S = 6, 8, 10, 12 and 14), were manufactured by using the optimal binder and two hemp shives (0–15 mm and 0–20 mm). Density of the starch-hemp composite materials varies with the H/S ratio. The dry density for the starch-hemp composite materials is lower, between 163.6 kg/m³ and 169.1 kg/m³ in case of the hemp shives 0–15 mm and between 168.1 kg/m³ and 174.3 kg/m³ for the hemp shives 0–20 mm. The relation between stress and strain of the composite materials is not linear. The ultimate compressive stress can reach 0.55 MPa and the compressive strain is up to 30%. The results obtained by test show that the tensile strength depends strongly on the Hemp/Starch ratio and the hemp shives sizes. The variation of elasticity modulus and Poisson's ratio in function of the H/S ratio was also analyzed in this paper. The mechanism of the cracks or failure of the specimens was studied by using ARAMIS optical system. The study on acoustical behavior shows that the starch-hemp composite materials are a good sound absorber material for medium and high frequencies with a value around 0.7. The influence of the H/S ratio on the absorption coefficient is small. The results show that the starch-hemp composite materials have a good mechanical and acoustical performance and can be used as building materials.     

The effects of surface elasticity and surface tension on the transverse overall elastic behavior of unidirectional nano-composites

The effects of surface elasticity and surface tension on the transverse overall behavior of unidirectional nano-scale fiber-reinforced composites are studied. The interfaces between the nano-fibers and the matrix are regarded as material surfaces described by the Gurtin and Murdoch model. The analysis is based on the equivalent inhomogeneity technique. In this technique, the effective elastic properties of the material are deduced from the analysis of a small cluster of fibers embedded into an infinite plane. All interactions between the inhomogeneities in the cluster are precisely accounted for. The results related to the effects of surface elasticity are compared with those provided by the modified generalized self-consistent method, which only indirectly accounts for the interactions between the inhomogeneities. New results related to the effects of surface tension are presented. Although the approach employed is applicable to all transversely isotropic composites, in this paper we consider only a hexagonal arrangement of circular cylindrical fibers.     

Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part II: Thermoplastic polyurethane matrix composites

A combined gel-casting and hot-pressing method was used to fabricate platelet-reinforced polymer matrix composites. Submicrometer thin alumina platelets were dispersed in a highly diluted polymer solution. A thermoplastic polyurethane elastomer was used as matrix for its high elasticity and excellent adhesion to the platelets. After dissolution of the polymer and casting, quick evaporation of the solvent triggered the formation of a polymer gel trapping the platelets in their well dispersed positions. The polymer–platelet gel densified during drying and the platelets were oriented horizontally due to the capillary forces and the large decrease in the thickness of the gel. The dried composites were hot-pressed to further improve the platelet orientation along the shear flow and close potential pores in the polymer. While the ultimate tensile strength of the composites gradually decreased with increasing platelet volume fractions, the increase in the elastic modulus and the stress necessary to deform the composite 10% was more than 100 and 18 times higher than the respective values of the pure polymer. The use of alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode. Since the polymer had to deform more to achieve identical deformation of the composite at higher platelet volume fraction, the strain at rupture steadily decreased. The incorporation of voids towards high platelet concentrations and the thereby triggered crack initiation and growth during straining lead to an additional decrease in the elasticity of composites with increasing platelet volume fractions. However, the extremely high extensibility of a polymer matrix allowed for the fabrication of composites that still deformed up to 162 ± 19% at platelet volume fractions as high as 0.33. When compared to other platelet-reinforced elastomers, the achieved platelet volume fraction is much higher and the relative increase in elastic modulus and stress at low strains is therefore much larger at the expense of a decrease in the strain at rupture. The fabrication method and designing principles employed in this study are transferable to other types of polymers and platelets and potentially allow the creation of new composites with tailored properties.     

Flammability of raw insulation materials made of hemp

Due to the fact that natural materials are more sensitive to flammability, it is necessary to determine flammability properties of nonwovens from natural fibers. This paper reports the fire reaction test results comparison of non-woven hemp fibers insulation materials made by three technologies. Hydro-entangling, thermal-bonding and needle-punching technologies were used for samples production from carded web. This review particularly compares the effects of flammability to find out influences of fibers properties and applied non-woven technologies.     

Mechanical behavior of Kevlar/basalt reinforced polypropylene composites

In this study, mechanical behavior of thermoplastic composites reinforced with two-dimensional plain woven homogeneous and hybrid fabrics of Kevlar/basalt yarns was studied. Five types (two homogeneous and three hybrids) of composite laminates were manufactured using compression molding technique with polypropylene (PP) resin. Static tensile and in-plane compression tests were carried out to evaluate the mechanical properties of the laminates. The tension and in-plane compression tests had shown that the composites with the combination of Kevlar and basalt yarns present better tensile and in-plane compressive behavior as compared to their base composites. Improvement in the properties such as elastic modulus, strength and failure strain in both tension and in-plane compression was observed due to the hybridization. Numerical simulations were performed in ABAQUS/Standard by implementing a user-defined material subroutine (VUMAT) based on Chang-Chang criteria. Good agreement between the experimental and numerical simulations was achieved in terms of damage patterns.     

Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

Notch effects and crack propagation analysis on kenaf/polypropylene nonwoven composites

This paper has studied the open-hole and pin filled-hole effects on the tensile properties of Kenaf/Polypropylene Nonwoven Composites (KPNCs) in production of automotive interior parts. The influence of specimen width-to-hole diameter (W/D) ratios of 6, 3, and 2 on failure load was studied. Two sample thicknesses of 3 mm and 6 mm were evaluated. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. A preliminary model by extended finite element method (XFEM) was established to predict the failure load and simulate crack propagation of 3 mm thick open-hole and pin filled-hole specimens. Good agreement was found between experimental and simulation results. By calculating the stress concentration factor K_t for brittle materials, the net section stress factor K_n for ductile materials, and the strength reduction factor K_r, it was found that KPNC was relatively ductile and insensitive to the notch.     

Comparative electromechanical damage-sensing behaviors of six strain-hardening steel fiber-reinforced cementitious composites under direct tension

This research investigates the electromechanical damage-sensing behavior of strain-hardening steel fiber-reinforced cement composites (SH-SFRCs) with six types of steel fibers (1.5% volume fraction content) within an identical mortar matrix (90 MPa). The six types of steel fibers studied are long twisted (T30/0.3), long smooth (S30/0.3), long hooked (H30/0.375), medium twisted (T20/0.2), medium smooth (S19/0.2), and short smooth (S13/0.2) steel fibers. The damage-sensing behavior was evaluated by measuring the changes in the electrical resistance during direct tensile tests. The electrical resistivity of the SH-SFRCs clearly decreased as the tensile strain increased until the post-cracking point, owing to the generation of multiple micro-cracks during strain-hardening. All the SH-SFRCs investigated had nominal gauge factors ranging between 50 and 140; these values are much higher than the commercially conventional gauge factor, which involves metal and is around 2. Both T30/0.3 and T20/0.2 produced the highest gauge factor, i.e., the best damage-sensing capacity, whereas S13/0.2 produced the highest electrical conductivity.     

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.     

Three consistent approaches of the multiple cracking process in 1D composites

Most modellings found in literature for the multiple cracking process of 1D composites can be categorised into three different approaches: a Continuous Approach (CA) that assumes an infinitely long composite, and two random approaches that consider composites of finite length. The Random Strength Approach (RSA) rests on a spatial discretization of the composite on which a strength distribution is applied, whereas the Random Crack Approach (RCA) generates the location and the strength of each new crack without any discretization.     

Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites

The objective of this study was to investigate the effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of hemp fibre/epoxy composites. Pectin removal by EDTA and endo-polygalacturonase (EPG) removed epidermal and parenchyma cells from hemp fibres and improved fibre separation. Hemicellulose removal by NaOH further improved fibre surface cleanliness. Removal of epidermal and parenchyma cells combined with improved fibre separation decreased composite porosity factor. As a result, pectin removal increased composite stiffness and ultimate tensile strength (UTS). Hemicellulose removal increased composite stiffness, but decreased composite UTS due to removal of xyloglucans. In comparison of all fibre treatments, composites with 0.5% EDTA + 0.2% EPG treated fibres had the highest tensile strength of 327 MPa at fibre volume content of 50%. Composites with 0.5% EDTA + 0.2% EPG → 10% NaOH treated fibres had the highest stiffness of 43 GPa and the lowest porosity factor of 0.04.     

Constitutive modelling of fibre-reinforced composites with unidirectional plies using a plasticity-based approach

This paper presents the development of a constitutive model able to accurately represent the full non-linear mechanical response of polymer-matrix fibre-reinforced composites with unidirectional (UD) plies under quasi-static loading. This is achieved by utilising an elasto-plastic modelling framework. The model captures key features that are often neglected in constitutive modelling of UD composites, such as the effect of hydrostatic pressure on both the elastic and non-elastic material response, the effect of multiaxial loading and dependence of the yield stress on the applied pressure.The constitutive model includes a novel yield function which accurately represents the yielding of the matrix within a unidirectional fibre-reinforced composite by removing the dependence on the stress in the fibre direction. A non-associative flow rule is used to capture the pressure sensitivity of the material. The experimentally observed translation of subsequent yield surfaces is modelled using a non-linear kinematic hardening rule. Furthermore, evolution laws are proposed for the non-linear hardening that relate to the applied hydrostatic pressure.Multiaxial test data is used to show that the model is able to predict the non-linear response under complex loading combinations, given only the experimental response from two uniaxial tests.     

Processing, compatibilization and properties of ternary composites of Mater-Bi with polyolefins and hemp fibres

Ternary composites of a biodegradable thermoplastic matrix, Mater-Bi® (MB), with various polyolefins (PP, HDPE and PS) and hemp fibres (H) were obtained by melt mixing and characterized by SEM, OM, DSC, TGA and tensile tests. The properties of composites were compared with those of MB/polyolefin and MB/H blends. Maleic anhydride functionalized polyolefins were employed as compatibilizers. Crystallization behaviour and morphology of the composites were found to be affected by the composition, phase dispersion and compatibilizer. Thermogravimetric analysis indicated that the thermal stability of the polyolefin phase and fibres was influenced by the composition and phase structure. A significant improvement of tensile modulus and strength was recorded for composites of MB with PE and PS as compared to MB/H composites. The results indicate that incorporation of polyolefins in the biodegradable matrix, compared to binary matrix/fibre system, may have significant advantages in terms of properties, processability and cost.